1. Field of the Invention
The invention relates to a fiber-optic current sensor having a reflection interferometer, a method for setting an operating point in a current sensor of this type, and a method for current measurement by means of a current sensor of this type.
2. Background Information
A fiber-optic current sensor of the generic type is disclosed by DE-A-4,224,190 and G. Frosio et al., “Reciprocal reflection interferometer for a fiber-optic Faraday current sensor”, Applied Optics, Vol. 33, No. 25, pages 6111–6122 (1994). The current sensor has a magneto-optically active sensor fiber which is wound in coil form and encloses a current conductor. The sensor fiber is mirror-coated at one end and, at the other end, it is connected via a phase delay element to a polarization-maintaining optical lead fiber via which light can be coupled into and out of the sensor fiber. In this case, the lead fiber propagates mutually orthogonally linearly polarized optical waves. With the aid of a fiber-optic phase retarder, said waves are converted into two circularly polarized waves before entering the sensor coil, the two circularly polarized waves having a mutually opposite rotation sense. After traversing the sensor coil, the two circular waves are reflected at the end of the coil, and they travel back through the coil with an interchanged polarization sense.
If a current then flows through the current conductor, the magnetic field of the current brings about a differential phase shift between the two circular optical waves. This effect is called the magneto-optical or Faraday effect. As a result of traversing the coil twice, the waves accumulate a differential phase shift of ΔΦs=4 V N I, where V denotes the Verdet's constant of the fiber, N denotes the number of fiber turns in the coil and I denotes the current through the current conductor.
Upon emerging from the coil, the circular waves are converted into orthogonal linearly polarized waves again in the phase retarder and are passed via the lead fiber to a detection system. The polarization directions of the returning orthogonal waves are interchanged compared to the forward-traveling waves. The phase shift caused by the current can be detected by the two reflected linearly polarized waves being brought to interference in a polarizer adjoining the lead fiber.
In order to achieve a useable interference signal, the effective operating point of the interferometer must be brought to a linear region of a cos-shaped interference function. This is done by means of a modulation unit with a phase modulator which alters the birefringence in the lead fiber and thus a differential phase of the two waves. Since both forward-traveling and backward-traveling waves pass through the same phase modulator, the latter has to oscillate with a frequency adapted to the circulation time of the waves, in order to modulate the differential phase of the two interfering waves non-reciprocally. Without a modulation unit, the phase difference between the two interfering waves would be equal to zero.
The modulation frequency ideally corresponds to the inverse value of twice the circulation time of the light in the interferometer. The frequency of the modulation typically lies in the range between 100 kHz and a few MHz and is determined inter alia by the length of the fiber connection to the sensor fiber, that is to say the lead fiber.
The current-induced phase shift can be determined by suitable demodulation. The demodulation techniques are the same ones which are used for fiber-optic gyroscopes and are described for example in R. A. Bergh et al. “An Overview of fiber-optic gyroscopes”, J. Lightwave Technol. 2, 91'107 (1984). In this case, a distinction is made essentially between open-loop and closed-loop configurations.
In order that the fiber-optic current sensor can be employed in practice, it requires a good long-term stability. Unfortunately, simple modulation units, for example those with piezoelectric modulators, have a drift in their amplitude, e.g. as a result of temperature changes. Therefore, in the prior art, use is made of relatively expensive modulation units which are as stable as possible and have integrated optical modulators or means for compensating fluctuations in amplitude. Such means are, for example, measuring means for determining the amplitude of the phase modulation in order to keep it constant by means of an additional regulating circuit. However, said means lead to a complicated construction of the sensor and thus increase the costs.